feat(opencode): add backend server selection options for acp mode

[georgeharker]

What does this PR do?

Allows app connections to specify a shared server backend for open code. This means you can have one server process, multiple acp connections.

How did you verify your code works?

Started server with opencode serve --port 4097 connected two acp clients to it from separate neovim by specifying opencode app --connect http://localhost:4097 for the launch lines of the acp clients in CodeCompanion.

Fixes #12853

[github-actions]

Hey! Your PR title add server options for acp mode doesn't follow conventional commit format.

Please update it to start with one of:

• feat: or feat(scope): new feature
• fix: or fix(scope): bug fix
• docs: or docs(scope): documentation changes
• chore: or chore(scope): maintenance tasks
• refactor: or refactor(scope): code refactoring
• test: or test(scope): adding or updating tests

Where scope is the package name (e.g., app, desktop, opencode).

See CONTRIBUTING.md for details.

[github-actions]

The following comment was made by an LLM, it may be inaccurate:

No duplicate PRs found

[github-actions]

Thanks for your contribution!

This PR doesn't have a linked issue. All PRs must reference an existing issue.

Please:

1. Open an issue describing the bug/feature (if one doesn't exist)
2. Add Fixes #<number> or Closes #<number> to this PR description

See CONTRIBUTING.md for details.

[georgeharker]

Added attached issue and renamed PR